(i) Field of the Invention
This invention relates to a hydraulic servo for friction coupling elements in automatic transmissions.
(ii) Description of the Prior Art:
Heretofore, hydraulic servos for friction coupling elements in automatic transmissions have included an annular drum having an outer cylinder, an inner cylinder and a side wall connecting the two cylinders, fixed or rotatably mounted in the transmission case, a piston provided between the outer and inner cylinders of the drum, and a return spring located outward of the outer cylinder or on a wall portion of the piston, coupling and releasing the friction element by supplying and draining line pressure to and from a chamber between the drum and piston.
In a case where the return spring is located outward of the outer cylinder, however, the hydraulic servo of the friction coupling element unavoidably has large radial dimension. On the other hand, the arrangement in which the return spring is mounted on a wall portion of the piston has a drawback in that the hydraulic servo of the friction coupling element becomes lengthy in the axial direction. Further, when upshifting a vehicular automatic transmission from first to second speed, the outer race of a one-way clutch F2 is fixed to the automatic transmission case by engaging a friction brake B2. In this connection, the hydraulic servo B-2 which engages or releases this brake B2 has an annular hydraulic drum and a piston which is biased by a return spring, usually involving a wide piston mounting space between the outer and inner cylinders of the hydraulic drum. Therefore, there is little space for mounting the return spring unless the case is extended to a certain extent to secure the return spring mounting space. In addition, shift shocks occur on upshifts from first to second speed due to the use of a large piston. In order to solve the problem of shift shocks, we previously proposed a hydraulic servo for friction coupling elements of automatic transmissions (Japanese Patent Application No. 59-89075), employing a piston of an optimum area for a friction coupling element to suppress the shift shocks and at the same time to save a space for mounting piston return springs, providing a hydraulic servo of a compact construction which can be mounted in position without extending its mounting space in the axial or radial direction.
In addition, there has been a problem that, when assembling a connecting member with a piston, it tends to lose stability in the absence of a guide means which guides the tip end of the connecting member.